Simón-Díaz, S.; Herrero, A.; Paredes, J. M.; Ribas, I.; Ribó, M.; Negueruela, I.; Casares, J.
Bibliographical reference
Nature, Volume 505, Issue 7483, pp. 378-381 (2014).
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2014
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Description
Stellar-mass black holes have all been discovered through X-ray
emission, which arises from the accretion of gas from their binary
companions (this gas is either stripped from low-mass stars or supplied
as winds from massive ones). Binary evolution models also predict the
existence of black holes accreting from the equatorial envelope of
rapidly spinning Be-type stars (stars of the Be type are hot blue
irregular variables showing characteristic spectral emission lines of
hydrogen). Of the approximately 80 Be X-ray binaries known in the
Galaxy, however, only pulsating neutron stars have been found as
companions. A black hole was formally allowed as a solution for the
companion to the Be star MWC656 (ref. 5; also known as HD215227),
although that conclusion was based on a single radial velocity curve of
the Be star, a mistaken spectral classification and rough estimates of
the inclination angle. Here we report observations of an accretion disk
line mirroring the orbit of MWC656. This, together with an improved
radial velocity curve of the Be star through fitting sharp FeII profiles
from the equatorial disk, and a refined Be classification (to that of a
B1.5-B2 III star), indicates that a black hole of 3.8 to 6.9 solar
masses orbits MWC656, the candidate counterpart of the γ-ray
source AGLJ2241+4454 (refs 5, 6). The black hole is X-ray quiescent and
fed by a radiatively inefficient accretion flow giving a luminosity less
than 1.6×10-7 times the Eddington luminosity. This
implies that Be binaries with black-hole companions are difficult to
detect in conventional X-ray surveys.
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